Is it permissible for the husband to masturbate first, then engage in marital intercourse afterward to prolong the duration of coitus, in light of his wife’s refusal to assist him in that?

Masturbation is forbidden and harmful. It is not permissible to engage in it except out of dire necessity, such as the fear of committing zina (unlawful sexual intercourse). Premature ejaculation does not justify masturbation, and it can be treated by consulting doctors. Masturbation with the wife's hand is permissible. If you cannot convince your wife to do it, then enjoy her in any other permissible way to release your sexual desire. It is permissible for a husband to enjoy his wife in all ways except for intercourse during menstruation and anal intercourse.